CHAPTER XXV
THE MOTH AND THE CANDLE
IN order to understand and interpret correctly the operation of natural selection in producing new species and maintaining them, by "the preservation of favoured races in the struggle for life" (to use Darwin's words), we must take a wide and, at the same time, a minutely accurate survey of the living world. We must seek out the evidences of this operation and use the imagination in forming conceptions as to the varied steps of the process and the results which are likely to ensue from it at different stages and in different conditions. We cannot interpret the existing structures and behaviour of living things by the use of a simple formula, such as that set up by some writers who have not properly studied Mr. Darwin's works, and declare that, according to him, all structures and behaviours which we observe in living things are perfect and the finished result of survival of the ideally fittest variations.
Plants and animals are so complex (as no one has shown more clearly than Darwin), not only in their structure but in the chemical and physical action and interaction of their living parts, that in the course of the ages during which the present species have been, step by step, fashioned in the endless vicissitudes of a changing world, many of them have retained structures or chemical constitutions which once were useful but now are useless, or even positively injurious. Even injurious structures or behaviours may be retained and inherited by a species of plant or animal, if, on the whole, the other accompanying modifications of structure are valuable—that is, of "life-saving" value, so that, "on the whole," the race is favoured by selection in the struggle for existence.
In species which have but lately acquired dominance or are brought by their success into novel conditions, we may, and do, find old structures and behaviours still persisting which are injurious, not yet, as it were, "cleaned up" and got rid of as they would be in the course of further long periods of selection. Such species become established, and may even acquire a definite stability, because the injurious structures or behaviours which they have retained are of little or no account as compared with the other advantageous characters which the species have developed. The term "disharmonies" is applied to such injurious characters, consisting in a certain want of harmony (in minor respects) between the structure of an organism and the conditions in which, nevertheless, it thrives.
Such species, imperfect because of their "disharmonies," are an illustration of the fact that Nature herself, in matters relating to living things, is not averse to compromise. Nature sets the example of toleration. Toleration may be defended on the ground that it is the biological method. Nature, though stern and inexorable as to essentials, yet accepts the faults and defects of some of her children because of the virtues and excellences which accompany them. The most highly endowed and successful forms on account of their dominance and power of spreading into new conditions, are even more likely than less highly developed kinds to retain concealed defects—disharmonies which do not lead to the destruction of the species, but occasionally cause strange embarrassment to it until they are, possibly in the long process of ages, got rid of by the slow operation of selection and survival of those individuals in which the injurious character varies in the direction of diminution and ultimate disappearance.
In man (owing, apparently, to the rapid rate at which he has been carried along towards dominance over the whole face of the globe by the development of his intelligence) the bodily structure has failed to keep pace with and to become perfected, "trimmed up," and completely adapted to, the newly-acquired habits which his increasing intelligence has forced on him. His "wisdom teeth" are "disharmonies." They are now useless and dwindled, weak spots open to the attacks of disease—since they are no longer needed for grinding coarse vegetable food, and are consequently no longer kept (by the speedy death of those individuals in whom they are small) at the full original size and efficiency seen in the apes. His large intestine is a "disharmony" not yet got rid of by natural selection, although no longer useful, but, on the contrary, the seat of poisonous putrefaction and absorption of such poisons. His tail—a few small vertebræ beneath the skin—is absolutely useless, and occasionally the seat of dangerous injury or disease. Tails very generally are liable to become useless in the descendants of animals in which they were invaluable as "fly-brushes" (cattle, horses, etc.), as prehensible organs (American monkeys), as concealing cloaks (South American ant-eater), as aids to swimming or flying, or as ornamental glories (the big-cats and others). The stumpy tail of the lynx, of some monkeys, and some lizards and fishes tells of a history in which the full-sized tail became a "disharmony"—a positive nuisance—and has been reduced, even if not abolished, by natural selection of short-tailed or tail-less varieties.
We have to be careful in asserting that any structure or behaviour in an organism is certainly a "disharmony," for it is very difficult to be quite sure as to the complete details of the life of a wild creature, and so to be able to form a conclusion rather than to suggest a possibility—as to the part played by an apparently injurious structure or habit in the economy of that creature.
One of the most striking instances of a habit or behaviour which persists and dominates the life of a wild animal to its own injury and destruction is that shown by many moths and other insects, which are attracted at night by a flame (a lamp or an open fire), and fly into it even when burnt by it, again and again until they are killed. A burnt child dreads the fire; but a burnt moth or a singed ichneumon fly seems to enjoy being burnt, and becomes more and more excited by its dashes into the flame until it finally drops with shrivelled wings to the ground. My brother told me some years ago of the verandah of his house in Java in which an open lamp was lit every night. Regularly two sets of animals, driven and guided by the action of the light on their nervous mechanism, arrived on the scene. Swarms of moths and flies dashed in and out of the flame and fell, maimed by the heat, to the ground. There a strange group had already assembled. Gigantic toads and wall lizards crept from their holes in the masonry and woodwork, and awaited the shower of injured insects, which they snapped up in eager rivalry as the infatuated flame-seekers dropped, hour after hour, to the floor. The instinct, the nervous mechanism, which brought the greedy reptiles to the spot was a "harmony," a valuable guide to nutrition; whilst the flame-seeker's impulse is assuredly a "disharmony"—a defect in adjustment—leading to death.
It is interesting to inquire into the probable origin of this fatal desire for close contact with a source of light, a desire so strong as to be entirely unchecked by the deadly heat accompanying the light. The May-flies or Ephemerids are delicate little creatures, having four net-veined wings rarely more than three-quarters of an inch across, with two or three long filaments hanging from the tail. Three hundred species are known from all parts of the world, of which forty occur in the British Islands. They live as wingless, six-legged larvæ in the water for a couple of years, feeding voraciously. Then one summer's evening they very rapidly escape from their larval skin and fly over the water in countless swarms. But only for a few hours. The eggs of the females are fertilized, and they all, both males and females, drop dead or dying into the water, where they are greedily devoured by fishes. The males are far more numerous than the females; in some species as many as 6000 males to one female have been counted. They are attracted to an extraordinary degree by lights (flames or electric lamps) set up for nocturnal illumination by civilized man, and in some districts they are collected by fishermen in this way for use as food for fish, or were so in Holland in the eighteenth century according to Swammerdam's statement in his "Biblia Naturæ." Why do they thus seek artificial lights? There is some indication of an explanation in the fact that two tropical species of May-flies are known which, like the glow-worms and fire-flies, produce light in their bodies. The May-flies, especially the males, have unusually large and prominent eyes, as is the case with phosphorescent fishes and some other light-producing animals, and it appears probable that in the now rare instances of self-luminous May-flies, the sexes are attracted to one another by the light they produce, as is the case in other luminous insects. It seems probable that the ancestral May-flies, of which many remarkable kinds have been discovered in the fossilized condition in strata as far back in time as those of the coal-measures, were all self-luminous, and acquired an overpowering instinct of seeking the light given out by other individuals as a necessary step towards sexual congress. In the course of ages other senses (probably smell and touch) have been called in to bring the fluttering insects into association. The power of producing light, being no longer needed, has disappeared from all but two rare species. But the urgent erotic instinct, the nervous mechanism, which drove the ancient May-flies towards the dancing lights of other May-flies, has remained unaltered in all the living species of the group. It is a "disharmony" which has not been of sufficient destructive importance to be "cleared away" or suppressed by natural selection. In pre-human times, nocturnal fires and lights were too uncommon to cause much disaster to the May-flies. But now that mankind sets up everywhere his nocturnal flames and electric lamps, the previously unimportant useless survival of an overpowering impulse to rush to nocturnal lights, reveals itself as a serious and death-dealing "disharmony." We must suppose, on this theory, that the other insects, such as moths and certain flies (by no means all insects), which also madly fly into nocturnal lights to their own destruction, have had luminous ancestors and a similar early history. This is a legitimate supposition, since there are several very distinct kinds of insects known at the present day which are luminous at night, although no existing moths or butterflies are known to be so.
A fact bearing on the explanation of the insects' perilous rush to flame is that birds when migrating are attracted by the great brilliant lamps of lighthouses, and, flying towards them, strike against their glass coverings, and are killed in considerable numbers. In that case, it may be that the flying towards the sun has become instinctive, and that the bright light of the lighthouse acts upon a certain number of birds (perhaps the less well-adjusted individuals) so as to call forth the same response in the direction of flight as that exercised by the sun's globe. The truth or error of this suggestion should be tested by an examination of the species of birds which kill themselves against lighthouse lanterns, and a knowledge of the season and direction of their migration.